1 //===- LazyCallGraph.h - Analysis of a Module's call graph ------*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
11 /// Implements a lazy call graph analysis and related passes for the new pass
14 /// NB: This is *not* a traditional call graph! It is a graph which models both
15 /// the current calls and potential calls. As a consequence there are many
16 /// edges in this call graph that do not correspond to a 'call' or 'invoke'
19 /// The primary use cases of this graph analysis is to facilitate iterating
20 /// across the functions of a module in ways that ensure all callees are
21 /// visited prior to a caller (given any SCC constraints), or vice versa. As
22 /// such is it particularly well suited to organizing CGSCC optimizations such
23 /// as inlining, outlining, argument promotion, etc. That is its primary use
24 /// case and motivates the design. It may not be appropriate for other
25 /// purposes. The use graph of functions or some other conservative analysis of
26 /// call instructions may be interesting for optimizations and subsequent
27 /// analyses which don't work in the context of an overly specified
28 /// potential-call-edge graph.
30 /// To understand the specific rules and nature of this call graph analysis,
31 /// see the documentation of the \c LazyCallGraph below.
33 //===----------------------------------------------------------------------===//
35 #ifndef LLVM_ANALYSIS_LAZYCALLGRAPH_H
36 #define LLVM_ANALYSIS_LAZYCALLGRAPH_H
38 #include "llvm/ADT/DenseMap.h"
39 #include "llvm/ADT/PointerUnion.h"
40 #include "llvm/ADT/STLExtras.h"
41 #include "llvm/ADT/SetVector.h"
42 #include "llvm/ADT/SmallPtrSet.h"
43 #include "llvm/ADT/SmallVector.h"
44 #include "llvm/ADT/iterator.h"
45 #include "llvm/ADT/iterator_range.h"
46 #include "llvm/IR/BasicBlock.h"
47 #include "llvm/IR/Function.h"
48 #include "llvm/IR/Module.h"
49 #include "llvm/IR/PassManager.h"
50 #include "llvm/Support/Allocator.h"
54 class PreservedAnalyses;
57 /// \brief A lazily constructed view of the call graph of a module.
59 /// With the edges of this graph, the motivating constraint that we are
60 /// attempting to maintain is that function-local optimization, CGSCC-local
61 /// optimizations, and optimizations transforming a pair of functions connected
62 /// by an edge in the graph, do not invalidate a bottom-up traversal of the SCC
63 /// DAG. That is, no optimizations will delete, remove, or add an edge such
64 /// that functions already visited in a bottom-up order of the SCC DAG are no
65 /// longer valid to have visited, or such that functions not yet visited in
66 /// a bottom-up order of the SCC DAG are not required to have already been
69 /// Within this constraint, the desire is to minimize the merge points of the
70 /// SCC DAG. The greater the fanout of the SCC DAG and the fewer merge points
71 /// in the SCC DAG, the more independence there is in optimizing within it.
72 /// There is a strong desire to enable parallelization of optimizations over
73 /// the call graph, and both limited fanout and merge points will (artificially
74 /// in some cases) limit the scaling of such an effort.
76 /// To this end, graph represents both direct and any potential resolution to
77 /// an indirect call edge. Another way to think about it is that it represents
78 /// both the direct call edges and any direct call edges that might be formed
79 /// through static optimizations. Specifically, it considers taking the address
80 /// of a function to be an edge in the call graph because this might be
81 /// forwarded to become a direct call by some subsequent function-local
82 /// optimization. The result is that the graph closely follows the use-def
83 /// edges for functions. Walking "up" the graph can be done by looking at all
84 /// of the uses of a function.
86 /// The roots of the call graph are the external functions and functions
87 /// escaped into global variables. Those functions can be called from outside
88 /// of the module or via unknowable means in the IR -- we may not be able to
89 /// form even a potential call edge from a function body which may dynamically
90 /// load the function and call it.
92 /// This analysis still requires updates to remain valid after optimizations
93 /// which could potentially change the set of potential callees. The
94 /// constraints it operates under only make the traversal order remain valid.
96 /// The entire analysis must be re-computed if full interprocedural
97 /// optimizations run at any point. For example, globalopt completely
98 /// invalidates the information in this analysis.
100 /// FIXME: This class is named LazyCallGraph in a lame attempt to distinguish
101 /// it from the existing CallGraph. At some point, it is expected that this
102 /// will be the only call graph and it will be renamed accordingly.
103 class LazyCallGraph {
107 typedef SmallVector<PointerUnion<Function *, Node *>, 4> NodeVectorT;
108 typedef SmallVectorImpl<PointerUnion<Function *, Node *>> NodeVectorImplT;
110 /// \brief A lazy iterator used for both the entry nodes and child nodes.
112 /// When this iterator is dereferenced, if not yet available, a function will
113 /// be scanned for "calls" or uses of functions and its child information
114 /// will be constructed. All of these results are accumulated and cached in
117 : public iterator_adaptor_base<iterator, NodeVectorImplT::iterator,
118 std::forward_iterator_tag, Node> {
119 friend class LazyCallGraph;
120 friend class LazyCallGraph::Node;
123 NodeVectorImplT::iterator E;
125 // Build the iterator for a specific position in a node list.
126 iterator(LazyCallGraph &G, NodeVectorImplT::iterator NI,
127 NodeVectorImplT::iterator E)
128 : iterator_adaptor_base(NI), G(&G), E(E) {
129 while (I != E && I->isNull())
136 using iterator_adaptor_base::operator++;
137 iterator &operator++() {
140 } while (I != E && I->isNull());
144 reference operator*() const {
146 return *I->get<Node *>();
148 Function *F = I->get<Function *>();
149 Node &ChildN = G->get(*F);
155 /// \brief A node in the call graph.
157 /// This represents a single node. It's primary roles are to cache the list of
158 /// callees, de-duplicate and provide fast testing of whether a function is
159 /// a callee, and facilitate iteration of child nodes in the graph.
161 friend class LazyCallGraph;
162 friend class LazyCallGraph::SCC;
167 // We provide for the DFS numbering and Tarjan walk lowlink numbers to be
168 // stored directly within the node.
172 mutable NodeVectorT Callees;
173 DenseMap<Function *, size_t> CalleeIndexMap;
175 /// \brief Basic constructor implements the scanning of F into Callees and
177 Node(LazyCallGraph &G, Function &F);
179 /// \brief Internal helper to insert a callee.
180 void insertEdgeInternal(Function &Callee);
182 /// \brief Internal helper to insert a callee.
183 void insertEdgeInternal(Node &CalleeN);
185 /// \brief Internal helper to remove a callee from this node.
186 void removeEdgeInternal(Function &Callee);
189 typedef LazyCallGraph::iterator iterator;
191 Function &getFunction() const {
195 iterator begin() const {
196 return iterator(*G, Callees.begin(), Callees.end());
198 iterator end() const { return iterator(*G, Callees.end(), Callees.end()); }
200 /// Equality is defined as address equality.
201 bool operator==(const Node &N) const { return this == &N; }
202 bool operator!=(const Node &N) const { return !operator==(N); }
205 /// \brief An SCC of the call graph.
207 /// This represents a Strongly Connected Component of the call graph as
208 /// a collection of call graph nodes. While the order of nodes in the SCC is
209 /// stable, it is not any particular order.
211 friend class LazyCallGraph;
212 friend class LazyCallGraph::Node;
215 SmallPtrSet<SCC *, 1> ParentSCCs;
216 SmallVector<Node *, 1> Nodes;
218 SCC(LazyCallGraph &G) : G(&G) {}
220 void insert(Node &N);
223 internalDFS(SmallVectorImpl<std::pair<Node *, Node::iterator>> &DFSStack,
224 SmallVectorImpl<Node *> &PendingSCCStack, Node *N,
225 SmallVectorImpl<SCC *> &ResultSCCs);
228 typedef SmallVectorImpl<Node *>::const_iterator iterator;
229 typedef pointee_iterator<SmallPtrSet<SCC *, 1>::const_iterator> parent_iterator;
231 iterator begin() const { return Nodes.begin(); }
232 iterator end() const { return Nodes.end(); }
234 parent_iterator parent_begin() const { return ParentSCCs.begin(); }
235 parent_iterator parent_end() const { return ParentSCCs.end(); }
237 iterator_range<parent_iterator> parents() const {
238 return iterator_range<parent_iterator>(parent_begin(), parent_end());
241 /// \brief Test if this SCC is a parent of \a C.
242 bool isParentOf(const SCC &C) const { return C.isChildOf(*this); }
244 /// \brief Test if this SCC is an ancestor of \a C.
245 bool isAncestorOf(const SCC &C) const { return C.isDescendantOf(*this); }
247 /// \brief Test if this SCC is a child of \a C.
248 bool isChildOf(const SCC &C) const {
249 return ParentSCCs.count(const_cast<SCC *>(&C));
252 /// \brief Test if this SCC is a descendant of \a C.
253 bool isDescendantOf(const SCC &C) const;
255 /// \brief Short name useful for debugging or logging.
257 /// We use the name of the first function in the SCC to name the SCC for
258 /// the purposes of debugging and logging.
259 StringRef getName() const { return (*begin())->getFunction().getName(); }
262 /// \name Mutation API
264 /// These methods provide the core API for updating the call graph in the
265 /// presence of a (potentially still in-flight) DFS-found SCCs.
267 /// Note that these methods sometimes have complex runtimes, so be careful
268 /// how you call them.
270 /// \brief Insert an edge from one node in this SCC to another in this SCC.
272 /// By the definition of an SCC, this does not change the nature or make-up
274 void insertIntraSCCEdge(Node &CallerN, Node &CalleeN);
276 /// \brief Insert an edge whose tail is in this SCC and head is in some
279 /// There must be an existing path from the caller to the callee. This
280 /// operation is inexpensive and does not change the set of SCCs in the
282 void insertOutgoingEdge(Node &CallerN, Node &CalleeN);
284 /// \brief Insert an edge whose tail is in a descendant SCC and head is in
287 /// There must be an existing path from the callee to the caller in this
288 /// case. NB! This is has the potential to be a very expensive function. It
289 /// inherently forms a cycle in the prior SCC DAG and we have to merge SCCs
290 /// to resolve that cycle. But finding all of the SCCs which participate in
291 /// the cycle can in the worst case require traversing every SCC in the
292 /// graph. Every attempt is made to avoid that, but passes must still
293 /// exercise caution calling this routine repeatedly.
295 /// FIXME: We could possibly optimize this quite a bit for cases where the
296 /// caller and callee are very nearby in the graph. See comments in the
297 /// implementation for details, but that use case might impact users.
298 SmallVector<SCC *, 1> insertIncomingEdge(Node &CallerN, Node &CalleeN);
300 /// \brief Remove an edge whose source is in this SCC and target is *not*.
302 /// This removes an inter-SCC edge. All inter-SCC edges originating from
303 /// this SCC have been fully explored by any in-flight DFS SCC formation,
304 /// so this is always safe to call once you have the source SCC.
306 /// This operation does not change the set of SCCs or the members of the
307 /// SCCs and so is very inexpensive. It may change the connectivity graph
308 /// of the SCCs though, so be careful calling this while iterating over
310 void removeInterSCCEdge(Node &CallerN, Node &CalleeN);
312 /// \brief Remove an edge which is entirely within this SCC.
314 /// Both the \a Caller and the \a Callee must be within this SCC. Removing
315 /// such an edge make break cycles that form this SCC and thus this
316 /// operation may change the SCC graph significantly. In particular, this
317 /// operation will re-form new SCCs based on the remaining connectivity of
318 /// the graph. The following invariants are guaranteed to hold after
319 /// calling this method:
321 /// 1) This SCC is still an SCC in the graph.
322 /// 2) This SCC will be the parent of any new SCCs. Thus, this SCC is
323 /// preserved as the root of any new SCC directed graph formed.
324 /// 3) No SCC other than this SCC has its member set changed (this is
325 /// inherent in the definition of removing such an edge).
326 /// 4) All of the parent links of the SCC graph will be updated to reflect
327 /// the new SCC structure.
328 /// 5) All SCCs formed out of this SCC, excluding this SCC, will be
329 /// returned in a vector.
330 /// 6) The order of the SCCs in the vector will be a valid postorder
331 /// traversal of the new SCCs.
333 /// These invariants are very important to ensure that we can build
334 /// optimization pipeliens on top of the CGSCC pass manager which
335 /// intelligently update the SCC graph without invalidating other parts of
338 /// The runtime complexity of this method is, in the worst case, O(V+E)
339 /// where V is the number of nodes in this SCC and E is the number of edges
340 /// leaving the nodes in this SCC. Note that E includes both edges within
341 /// this SCC and edges from this SCC to child SCCs. Some effort has been
342 /// made to minimize the overhead of common cases such as self-edges and
343 /// edge removals which result in a spanning tree with no more cycles.
344 SmallVector<SCC *, 1> removeIntraSCCEdge(Node &CallerN, Node &CalleeN);
349 /// \brief A post-order depth-first SCC iterator over the call graph.
351 /// This iterator triggers the Tarjan DFS-based formation of the SCC DAG for
352 /// the call graph, walking it lazily in depth-first post-order. That is, it
353 /// always visits SCCs for a callee prior to visiting the SCC for a caller
354 /// (when they are in different SCCs).
355 class postorder_scc_iterator
356 : public iterator_facade_base<postorder_scc_iterator,
357 std::forward_iterator_tag, SCC> {
358 friend class LazyCallGraph;
359 friend class LazyCallGraph::Node;
361 /// \brief Nonce type to select the constructor for the end iterator.
367 // Build the begin iterator for a node.
368 postorder_scc_iterator(LazyCallGraph &G) : G(&G) {
369 C = G.getNextSCCInPostOrder();
372 // Build the end iterator for a node. This is selected purely by overload.
373 postorder_scc_iterator(LazyCallGraph &G, IsAtEndT /*Nonce*/)
374 : G(&G), C(nullptr) {}
377 bool operator==(const postorder_scc_iterator &Arg) const {
378 return G == Arg.G && C == Arg.C;
381 reference operator*() const { return *C; }
383 using iterator_facade_base::operator++;
384 postorder_scc_iterator &operator++() {
385 C = G->getNextSCCInPostOrder();
390 /// \brief Construct a graph for the given module.
392 /// This sets up the graph and computes all of the entry points of the graph.
393 /// No function definitions are scanned until their nodes in the graph are
394 /// requested during traversal.
395 LazyCallGraph(Module &M);
397 LazyCallGraph(LazyCallGraph &&G);
398 LazyCallGraph &operator=(LazyCallGraph &&RHS);
401 return iterator(*this, EntryNodes.begin(), EntryNodes.end());
403 iterator end() { return iterator(*this, EntryNodes.end(), EntryNodes.end()); }
405 postorder_scc_iterator postorder_scc_begin() {
406 return postorder_scc_iterator(*this);
408 postorder_scc_iterator postorder_scc_end() {
409 return postorder_scc_iterator(*this, postorder_scc_iterator::IsAtEndT());
412 iterator_range<postorder_scc_iterator> postorder_sccs() {
413 return iterator_range<postorder_scc_iterator>(postorder_scc_begin(),
414 postorder_scc_end());
417 /// \brief Lookup a function in the graph which has already been scanned and
419 Node *lookup(const Function &F) const { return NodeMap.lookup(&F); }
421 /// \brief Lookup a function's SCC in the graph.
423 /// \returns null if the function hasn't been assigned an SCC via the SCC
425 SCC *lookupSCC(Node &N) const { return SCCMap.lookup(&N); }
427 /// \brief Get a graph node for a given function, scanning it to populate the
428 /// graph data as necessary.
429 Node &get(Function &F) {
430 Node *&N = NodeMap[&F];
434 return insertInto(F, N);
438 /// \name Pre-SCC Mutation API
440 /// These methods are only valid to call prior to forming any SCCs for this
441 /// call graph. They can be used to update the core node-graph during
442 /// a node-based inorder traversal that precedes any SCC-based traversal.
444 /// Once you begin manipulating a call graph's SCCs, you must perform all
445 /// mutation of the graph via the SCC methods.
447 /// \brief Update the call graph after inserting a new edge.
448 void insertEdge(Node &Caller, Function &Callee);
450 /// \brief Update the call graph after inserting a new edge.
451 void insertEdge(Function &Caller, Function &Callee) {
452 return insertEdge(get(Caller), Callee);
455 /// \brief Update the call graph after deleting an edge.
456 void removeEdge(Node &Caller, Function &Callee);
458 /// \brief Update the call graph after deleting an edge.
459 void removeEdge(Function &Caller, Function &Callee) {
460 return removeEdge(get(Caller), Callee);
465 /// \brief Print out the CFG to the provided stream.
467 /// This will fully traverse the call graph (and so is non-const) and print
468 /// it out to the provided stream.
469 void print(raw_ostream &OS, Module &M);
472 /// \brief Allocator that holds all the call graph nodes.
473 SpecificBumpPtrAllocator<Node> BPA;
475 /// \brief Maps function->node for fast lookup.
476 DenseMap<const Function *, Node *> NodeMap;
478 /// \brief The entry nodes to the graph.
480 /// These nodes are reachable through "external" means. Put another way, they
481 /// escape at the module scope.
482 NodeVectorT EntryNodes;
484 /// \brief Map of the entry nodes in the graph to their indices in
486 DenseMap<Function *, size_t> EntryIndexMap;
488 /// \brief Allocator that holds all the call graph SCCs.
489 SpecificBumpPtrAllocator<SCC> SCCBPA;
491 /// \brief Maps Function -> SCC for fast lookup.
492 DenseMap<Node *, SCC *> SCCMap;
494 /// \brief The leaf SCCs of the graph.
496 /// These are all of the SCCs which have no children.
497 SmallVector<SCC *, 4> LeafSCCs;
499 /// \brief Stack of nodes in the DFS walk.
500 SmallVector<std::pair<Node *, iterator>, 4> DFSStack;
502 /// \brief Set of entry nodes not-yet-processed into SCCs.
503 SmallVector<Function *, 4> SCCEntryNodes;
505 /// \brief Stack of nodes the DFS has walked but not yet put into a SCC.
506 SmallVector<Node *, 4> PendingSCCStack;
508 /// \brief Counter for the next DFS number to assign.
511 /// \brief Helper to insert a new function, with an already looked-up entry in
513 Node &insertInto(Function &F, Node *&MappedN);
515 /// \brief Helper to update pointers back to the graph object during moves.
516 void updateGraphPtrs();
518 /// \brief Helper to form a new SCC out of the top of a DFSStack-like
520 SCC *formSCC(Node *RootN, SmallVectorImpl<Node *> &NodeStack);
522 /// \brief Retrieve the next node in the post-order SCC walk of the call graph.
523 SCC *getNextSCCInPostOrder();
526 // Provide GraphTraits specializations for call graphs.
527 template <> struct GraphTraits<LazyCallGraph::Node *> {
528 typedef LazyCallGraph::Node NodeType;
529 typedef LazyCallGraph::iterator ChildIteratorType;
531 static NodeType *getEntryNode(NodeType *N) { return N; }
532 static ChildIteratorType child_begin(NodeType *N) { return N->begin(); }
533 static ChildIteratorType child_end(NodeType *N) { return N->end(); }
535 template <> struct GraphTraits<LazyCallGraph *> {
536 typedef LazyCallGraph::Node NodeType;
537 typedef LazyCallGraph::iterator ChildIteratorType;
539 static NodeType *getEntryNode(NodeType *N) { return N; }
540 static ChildIteratorType child_begin(NodeType *N) { return N->begin(); }
541 static ChildIteratorType child_end(NodeType *N) { return N->end(); }
544 /// \brief An analysis pass which computes the call graph for a module.
545 class LazyCallGraphAnalysis {
547 /// \brief Inform generic clients of the result type.
548 typedef LazyCallGraph Result;
550 static void *ID() { return (void *)&PassID; }
552 static StringRef name() { return "Lazy CallGraph Analysis"; }
554 /// \brief Compute the \c LazyCallGraph for the module \c M.
556 /// This just builds the set of entry points to the call graph. The rest is
557 /// built lazily as it is walked.
558 LazyCallGraph run(Module &M) { return LazyCallGraph(M); }
564 /// \brief A pass which prints the call graph to a \c raw_ostream.
566 /// This is primarily useful for testing the analysis.
567 class LazyCallGraphPrinterPass {
571 explicit LazyCallGraphPrinterPass(raw_ostream &OS);
573 PreservedAnalyses run(Module &M, ModuleAnalysisManager *AM);
575 static StringRef name() { return "LazyCallGraphPrinterPass"; }